Plastic waste is a growing global problem. The negative impact of plastic debris, particularly on marine ecosystems, has been appreciated for decades. But potentially, an even more significant danger is the insidious microplastic we cannot see. Plastic waste does not biodegrade, rather, it breaks down, fragmented by wind, waves, and sunlight into ever-smaller pieces.

Microplastics, defined as plastic fragments less than 5 mm in size, are everywhere: in aquatic ecosystems, indoor and outdoor air, bottled drinking water and food. A recent report documenting the universal presence of microplastics in stool samples from people in eight countries confirms that microplastics are also in us.

These findings raise significant concerns and questions about whether and how microplastics impact human health. But, as confirmed by three high-profile scientific reviews published earlier in 2019, we know remarkably little about their human effects.

A report by the Science Advice for Policy by European Academies consortium concluded that “we have no evidence of widespread risk to human health from micro and nanoplastics at present” but noted that “little is known… and what is known is surrounded by considerable uncertainty.”

A second report by the European Commission’s Group of Chief Scientific Advisors came to a similar conclusion: “Research on microplastics and their potential threats to humans is in its infancy and is complex – a lot remains uncertain.” The third, by the World Health Organization (WHO), which focused on the human health hazard of microplastics in drinking water, concluded that “….there is no evidence to indicate a human health concern”, but described the evidence as “limited” and “insufficient.”

At this time we do not know whether microplastics constitute a human health concern. However, emerging data suggests we should not assume that microplastics are safe.

Where do microplastics come from?

Plastics are synthetic carbon-based chains (polymers) composed of smaller repeating units (monomers), which often contain additives that influence durability and rigidity. One of the major challenges in characterising the toxicity of microplastics is that their physical and chemical characteristics are extremely diverse. In terms of size, microplastics span at least seven orders of magnitude, from 5 mm down to a nanometre. Their shape varies from perfect spheres to jagged shards and fibres.

Chemically, microplastics consist of hundreds or even thousands of different polymers, and varying types and amounts of plasticisers or flame retardants. Microplastics adsorb chemical contaminants from the environment, such as pesticides, polycyclic aromatic hydrocarbons, and even metals, and each particle is surrounded by a “biofilm” of microorganisms. Like snowflakes, every microparticle is unique.

There are multiple sources of microplastics. Primary microplastics are purposefully manufactured as industrial abrasives or microbeads used in cosmetics. Secondary microplastics result from the physical breakdown of plastic items through mechanical stress. Leading sources of environmental microplastics include clothing made from synthetic textiles, automotive tyre wear, city dust, road markings, coatings on ships and personal care products. Everyday activities also generate microplastics: a single load of laundry can release millions of microplastic fibres into wastewater. By some estimates, the average household generates 6 kilograms of plastic dust containing 700 billion plastic fragments every year.

Potential health hazards

The two main routes of human exposure to microplastics are ingestion and inhalation. But how much is taken up from the gut or lungs into the rest of the body remains unknown. Animal studies demonstrate that ingested microplastics are distributed to other organ systems. But the relevance of these findings to humans is questioned because the concentration of microplastics administered to animals was much higher than is found in the human diet.

Theoretically, microplastics pose physical, chemical, and microbiological hazards.

The physical hazards of large plastic debris are well documented in aquatic species, including marine mammals, sea turtles, and marine birds. These effects include entrapment of animals in plastic debris, malnourishment due to ingestion of plastics in lieu of food, and obstruction of the gastrointestinal tract when plastic is mistakenly consumed. Microplastics can have similar physical effects on smaller organisms at the base of many food webs. Negative effects on “primary producers” in aquatic ecosystems, such as algae and zooplankton, could lead to larger ecosystem disturbance. That microparticles pose a physical hazard to humans seems possible based on what we know about nanoparticles. For example, it is well documented that inhaled submicron particles cause physical irritation and inflammation in the lung.

Chemical hazards are the toxic effects of chemicals released from microparticles. Polymerisation reactions during production do not generally proceed to completion, thus, plastics often contain small proportions of monomers, such as 1,3-butadiene, ethylene oxide, and vinyl chloride, that can leach out of plastic. These chemicals are known to cause neurotoxicity, lung and liver damage and cancer. Many additives, which typically are not covalently bound to the polymer and thus readily leach out, have well-established toxicities. Toxic pollutants adsorbed to microparticles can also be released. The key question is whether any of these chemicals would reach concentrations in human tissues high enough to cause chronic toxicity following ingestion or inhalation of microparticles.

Microplastics can also serve as vectors for harmful microorganisms that readily colonise their surface. In this way, microplastics may concentrate or transport disease-associated bacteria and viruses, as well as filamentous cyanobacteria that cause harmful algal blooms. Microplastics may also facilitate the spread of antibiotic resistance genes.

Whether any of these hazards are relevant to human health has yet to be determined. However, ongoing studies in animal models and human cells suggest that microplastics may cause immune dysfunction and inflammation, raising questions about the role of microplastics in the epidemic of chronic inflammatory diseases.

Future steps

There are large gaps in our understanding of microplastics. Despite this, it is imperative we take immediate action to ban single-use plastic items and eliminate microbeads from cosmetics. Even these steps will not prevent a significant increase in microplastic pollution in the upcoming decades because of the vast amount of plastic waste already in the environment. Therefore, it is critical to invest in research now to rigorously evaluate the risks microplastics pose to humans and the environment.